Ethylene vinyl acetate (EVA) copolymers are commodity materials with weight percentages of vinyl acetate that usually varies from 2 to 40%. EVA copolymers are comparable to elastomeric materials in softness and flexibility, yet EVA copolymers can be processed like other thermoplastics. EVA copolymers have good clarity and gloss, good barrier and water proof properties, low-temperature toughness, desirable sealant properties, and resistance to UV radiation. They are inherently tough, resilient and more flexible than low density polyethylene over a broad temperature range, and have excellent environmental stress crack resistance.
Polyethylene (PE), Ethylene vinyl acetate (EVA), and polypropylene (PP) are common resins in packaging film and medical device applications. EVA is a common sealant in film packaging and is currently used extensively in primary packaging applications, including both film-film and paper-film applications, as well as components of medical devices. Polypropylene can be used as the structural component of packaging films and as components of many medical devices.
Polymer films for medical device and packaging must meet a broad range of stringent criteria which include: a) functional requirements of the product post-sterilization; b) providing a sterile barrier and structural support for the product over its lifetime after sterilization when used in primary packaging applications; c) being capable of high fabrication rates with a broad fabrication window; d) being cost effective; and e) meeting increasing demands for environmental stewardship.
Beyond the United States and European countries, radiation based sterilization techniques are not readily available and ethylene oxide (EtO) sterilization is the primary mode of sterilization used. A breathable package is required for EtO sterilization. Additionally, non-breathable film-film packages are currently limited with respect to the geographical markets they can be used due to limitations of the altitude the package can experience without potentially incurring open seals. At high altitudes the air within the non-breathable package expands and can cause open seals, resulting in loss of sterility of the product. Paper top webs are common alternatives used in packaging; however, they usually require an adhesive coating which increases the cost. Moreover, paper is susceptible to tearing and punctures, which can result in the loss of sterility of the product, and possible product recalls. Direct seal paper packaging is paper packaging without an adhesive coating. Although direct seal paper packaging is a low cost alternative, it is difficult to process on current packaging machines and can have a narrow seal performance window between weak, open seal or strong seal which result in fiber tear or tearing of the paper. Both the weak seal and paper tears of direct seal paper packaging compromise the sterility of the product. Breathable non-paper films, such as Tyvek®, can be used but they are substantially more costly than conventional films. PEBAX® is a commercially available polyamide/polymer ether copolymer which offers breathability and steam sterilization capabilities while maintaining a sterile barrier. However, like Tyvek®, PEBAX® is a specialized and expensive material.
An approach to achieving breathability of a polymeric film is the concept of micro-perforated films, which is utilized in a wide variety of commercial applications, commonly in food packaging and medical & health applications. The food packaging applications primarily apply to fresh product wrapping to enhance shelf life, but are also used in wrapping things such as fresh breads and other baked goods. These micro-perforated films are tailored to have very selective permeation rates to oxygen, carbon dioxide, and moisture. The controlled moisture vapor transmission rate (MVTR) preserves the moisture of the produce and extends its sellable shelf life. Alternatively, perforated films are also used in medical and health applications. These films typically have larger perforations thus resulting in poor physical properties and very high MVTR. These films include applications like breathable sheets for diapers and feminine hygiene products, wound dressings, exam and surgery room paper, etc. These films would not have the proper breathability to maintain a microbial barrier or the required physical properties for primary packaging of medical devices.
Other approaches to achieving breathability in packaging film are the use of rigid fillers, such as talc, and application of a post-fabrication stretching to the film. However, the use of rigid fillers results in a film having poor structural integrity and pores which would not maintain a microbial barrier. Moreover, the residual rigid filler would contaminate the medical device within the package.
It is desirable to incorporate functional properties onto known polymers to provide desired traits, such as breathability. However, the incorporation of novel chemistries along the polymer chain backbone cannot readily be achieved using known addition polymerization processes of polyethylene or polypropylene without a complicated secondary reactive process or via a step-growth process. The secondary chemical modification processes are often not 100% effective or chemically pure, resulting in incomplete and undesirable secondary reactions which detrimentally alter the ultimate chemical, thermal, and physical properties of the final polymer. The chain-growth process, the process by which block copolymers are produced, is a multi-step process which is complex, time consuming, and costly. While high levels of molecular homogeneity, including a relatively narrow dispersity index, can be achieved on a laboratory scale, large scale commercial processes using known addition polymerization process of polyethylene produce a mix of mono- and various multi-block structures and a broad dispersity index.
Thus, there is a need for a process capable of modifying EVA and maleic anhydride grafted isotactic polypropylene copolymers by allowing the incorporation of amphiphilic side chains onto the polymer chain backbone at high levels of molecular homogeneity, including a relatively narrow dispersity index. There is also a need for an improved thermoplastic elastomer or commodity resin that would allow for ethylene oxide sterilization in practical packaging and medical device applications.